Combustion System

ABSTRACT

Combustion system. The system includes a combustor generating a combustion gas and a splitter for receiving the combustion gas and dividing the combustion gas into first and second gas streams. A temperature controller receives the first gas stream, mixes it with recycled gas and introduces it into a heat recovery steam generator at a desired temperature. A mixer receives the second gas stream, mixes it with recycled gas and introduces it into the heat recovery steam generator. The cool gas that exits the heat recovery steam generator forms the recycled gas. A suitable combustor is an oxy-coal combustor.

This application claims priority to provisional application Ser. No.61/880266 filed on Sep. 20, 2013, the contents of which are incorporatedherein by reference.

This invention was made with government support under Grant No.DE-FE0009478 awarded by the Department of Energy. The government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention is a combustion system in which hot flue gases aresplit into at least two streams, mixed with recycled gas to control thegas temperature and introduced into a heat recovery steam generator. Thesplit arrangement allows for lower recycling power requirements and asmaller heat exchange area.

Reducing the capital cost and/or increasing the efficiency of powergeneration is highly desirable especially given the ever growing marketfor electric power [1]. Besides economic concerns, efficiency in powergeneration is also important because the dominating majority of electricenergy production is from non-renewable fossil fuels, and there areincreasing concerns and regulations regarding the associated greenhousegas emissions [2]. Flexibility to uncertain parameters, like fuelspecifications, ambient conditions, and thermal load is anotherimportant characteristic required from the implemented technologies.

In thermal power generation there are several forms of unavoidablelosses as well as several operations constraints and economicconsiderations. For example, heat exchanger areas are limited by capitalcost considerations, resulting in less effective heat transfer and/orlarger pressure drops of the streams due to the packed and constrainedpathways; thus, the efficiency of the cycle decreases. Metallurgicproperties pose temperature constraints on combustors, boilers, heatrecovery steam generators (HRSG), turbines, etc., [3,4], thus resultingin energy waste and reduction of power output. For example, flue gasrecycling (FGR) in coal boilers is required in order to quench thecombustion gas to limit the radiative-dominant (high temperature) heattransfer requirement and enable a convective-dominant heat transfer;flue gas recycling (FGR) is applied because radiation is more expensivethan convection for the same degree of thermal energy transfer, [5], PGRalso increases the boiler efficiency and reduces emissions, and isapplied in almost all relatively recent (younger than 30 years old) coalfired powerplants, [5, 6]. The FGR requires compression to compensatefor losses in the boiler and the recycling pipes; moreover, throughoutthe heat exchange process, the temperature gradient between the hot andcold streams decreases as flue gas moves away from the inlet, whichincreases the heat exchange area required close to the cold end of theheat exchanger.

One of the promising concepts of carbon-capture and sequestration ispressurized oxy-coal combustion (OCC). Pressurizing the flue gasincreases the effectiveness of the convection heat transfer, In [7, 8,9, 10, 11, 12, 13, 14], a pressurized OCC concept is considered with anHRSG with relatively high FGR that relies on connective heat transfer.Note that in general the combustion process occurs in a section that mayor may not be physically connected to the HRSG, referred to as acombustor. The capital cost of the HRSG is a relatively large portion ofthe capital cost of the powerplant, e.g., [15], and reducing its sizeresults in significant savings.

SUMMARY OF THE INVENTION

The combustion system of the invention includes a combustor generating acombustion gas and a splitter for receiving the combustion gas anddividing the combustion gas into first and second gas streams. Atemperature controller receives the first gas stream, mixes it withrecycled gas and introduces it into a heat recovery steam generator at adesired temperature. A mixer receives the second gas stream mixing itwith recycled gas and introducing it into the HRSG. Cool gas exits theHRSG with the cool gas forming the recycled gas. In a preferredembodiment, the combustor is an oxy-coal combustor. A preferredembodiment further includes recycling fans to pressurize the recycledgas for mixing with the first and second gas streams. The mass flow rateof the second gas stream, temperature at an inlet of the HRSG and flowrate of the recycled gas are controlled to optimize the system, tominimize heat exchanger area. Recycling power requirements are alsosmaller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a heat recovery steam generator aspart of a pressurized OCC process with a thermal recovery unit.

FIG. 2 is a graph of temperature versus heat recovery steam generatorduty transfer showing temperature profiles of four different operationsof flue gas with identical cold streams.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Herein, a novel split concept for the HRSG is introduced in order toenhance the rate of thermal energy transfer by increasing the averagetemperature between the involved streams, and reduce the compressionrequirements by reducing the recycling flow rates and/or pressure lossescompared to the conventional operation. The concept is applicable tocoal boilers and other heat exchange processes that require quenching ofthe hot fluid.

FIG. 1 illustrates the split concept applied to the HRSG of apressurized OCC process. [11, 12, 13, 14]. In pressurized OCC, oxygen isdelivered to the combustor at an elevated pressure. Primary recyclingflue gas, FG-Rec-pri, is mixed with the oxygen stream for dilution inorder to control the temperature of the combustor to acceptable levels,[3]; a temperature of 1550° is considered here. This temperature isentirely examplary. Combustion gas 10 is mixed with secondary recyclingflue gas, FG-Rec-sec, to achieve an acceptable temperature at the entryof the HRSG 12. In the HRSG 12, thermal energy is transferred to theworking fluid 14 of a Rankine cycle. Flue gas is recycled forcontrolling the critical temperature of the combustors and the HRSG intwo possible configurations, wet or dry recycling. In wet recycling fluegas is recycled directly after the HRSG exit, while in dry recyclingflue gas is recycled after condensing and separating the water. FIG. 1illustrates the wet recycling case, but the split design disclosedherein can be equivalently applied to dry recycling. Recycling fans 16are used to compensate for the pressure losses encountered by the fluegas mainly in the HRSG and the recycling pipes.

The concept disclosed herein splits the hot combustion gas prior to itsdilution by the secondary recycling stream. The flue gas entering theHRSG 12 decreases in temperature as it exchanges thermal energy with theworking fluid 14 of the Rankine cycle; flue gas acid condensation in theHRSG is not allowed as discussed later. In essence, the mixing of thesplit stream in the HRSG 12 is intended to elevate the flue gastemperature. The primary flue gas is mixed with a recycling streamsimilar to the HRSG without splitting. The split flue gas is(potentially) mixed with another secondary recycling stream. Thesecondary recycling to the split allows larger ranges and largerfeasible combinations of the mixing positions and the mixingtemperatures. The specific amount of recycling to the split, if any, isdetermined by optimization. Minimal area does not require any recyclingto the split, whereas minimal compensation power is required. Theadditional split pipe, the pipe of the recycling to the split (if used),and the recycling to the split recycling fan (if used) add some capitalcost, however, it is insignificant compared to the savings in the HRSG.Note that even when two recycling streams are used, a single fan 16 canbe installed by introducing some throttling at the outlet of the splitrecycling stream; obviously this results in higher compensation powerrequirements. For a given HRSG thermal load, the splitting process canincrease the overall temperature difference between the streams of theexchanger, particularly avoiding small temperature differences whichrequire the most heat transfer area. Moreover, the split allows forlower rates of recycled flue gas compared to regular HRSG: the requiredamount of recycling to the inlet of the HRSG is smaller because it needsto dilute a smaller amount of the combustion gas. Also, at the mixingposition, the flue gas within the HRSG acts as a diluent to the splitstream, thus a relatively small amount of the recycling to the split isrequired (if any).

The higher average temperature differences imply smaller exchanger area,and the lower recycling requirements imply lower pressure drops andlower compensation power requirements (CPR). The CPR consists of twocomponents; the first is the power needed by the recycling fans 16 tore-pressurize the recycling streams to compensate for the pressurelosses encountered in the HRSG 12 and the recycling pipes. The secondcomponent of the CPR is the power needed to maintain the main flue gasflow as it faces pressure losses while passing through the HRSG. Forsimplicity, only one split is illustrated and tested here. Multiplesplits, that would be introduced sequentially at different intermediatelocations, are possible and would further increase performance anddecrease the heat transfer area, but would add structural complexity.Note that in general the split can be extracted from any point along theHRSG and not limited to the inlet combustion gas, and the recycling canalso be withdrawn from any point along the HRSG and not limited to theoutlet Cool-Gas; however, such processes are less practical to implementand more complicated to model and optimize.

To illustrate the possible advantages, FIG. 2 shows the profiles of fourcases of HRSG operation as a standalone unit with an identical set ofinput parameters. As shown in Table 1, the input parameters are thespecifications of the hot and the cold input streams, the outletspecifications of the cold stream, and fractional thermal losses; thefixed specifications signify the total amount of thermal energytransferred in the HRSG is fixed. The numbers in Table 1 are exemplaryonly. Table 1 shows two sets of input parameters as obtained from anoptimized pressurized OCC with wet recycling, [13, 14]; thespecifications titled CoalA are the operating conditions that the HRSGencounters when the basecase OCC process utilizes a high quality coal,while those titled CoalB are relevant to combusting a lower qualitycoal, where CoalA and CoalB are identical to those utilized in [13].Both sets of input specifications of the streams are for the nominalfull load operation. The cold stream profile is that of the main icedwater and the reheat streams. The maximum allowed HRSG temperature is800° C. equal to that considered in [11, 12, 13, 14]. Similar to thebasecase the HRSG is considered to face a 0.75% fractional heat dutylosses. First, note that in FIG. 2 although the input streamsspecifications. CoalA specifications of Table 1, and the total, dutytransfer in the HRSG are constant among all four operations, thetemperature of the flue gas at the exit of the HRSG, Cool-Gas, might notbe. The different pressure losses and recycling requirements for eachoperation result in different CPR which causes different amounts ofcompression enthalpy rise (CER) carried by the flue gas, [11];therefore, the temperature of the Cool-Gas exiting the HRSG is slightlydifferent between the four profiles.

TABLE 1 Fixed input parameters for the HRSG. Two different flue gasconditions are presented, each relevant to a different coal type. InputParameter With CoalA With CoalB Flue Gas Conditions Combustion gasflowrate 402 kg/s 394 kg/s Combustion gas temperature 1550° C. Combustion gas pressure 7.41 bar 9.67 bar Combustion gas mole fractionH₂O = 0.479; O₂ = 0.030; H₂O = 0.478; O₂ = 0.030; Composition N₂ =0.008; CO₂ = 0.457; N₂ = 0.009; CO₂ = 0.458; SO₂ = 0.001; AR = 0.025 SO₂= 0.001; AR = 0.024 Maximum allowed HRSG temperature 800° C. HRSGfractional heat loss 0.75% Flue Gas flowrate to recovery unit 120 kg/s132 kg/s Feedwater and Reheat Conditions Feedwater flowrate 306 kg/s 301kg/s Feedwater inlet temperature 322° C. 322.0° C. Feedwater inletpressure  265 bar Feedwater outlet temperature 600° C. Feedwater outletpressure  250 bar Reheat flowrate 233 kg/s 260 kg/s Reheat inlettemperature 358° C. Reheat inlet pressure  53.5 bar Reheat outlettemperature 610° C. Reheat outlet pressure  53.1 bar CompressorsSpecifications Primary recycling fan η_(sentropic) = 0.83 η_(mechanical)= 0.99 Thermal spec = Adiabatic Secondary recycling fan0&1 η_(sentropic)= 0.8338 η_(mechanical) = 0.99 Thermal spec = Adiabatic Main streamcompensation compressor η_(sentropic) = 0.90 η_(mechanical) = 0.98Thermal spec = Adiabatic

The first profile in FIG. 2 is without splitting or recycling, i.e., allthe combustion gas enters the exchange directly without dilution; thisoperation violates the maximum temperature constraint on the HRSG and isonly shown for illustration. The infeasible operation of Profile 1.theoretically requires the smallest heat exchanger area due to thelargest temperature differences between the hot and cold streams, andrequires zero recycling and zero second component of the CPR.

The second profile in FIG. 2 represents the standard basecase operationwhere all the combustion gas is mixed with enough recycling to result inHot-Gas entering the HRSG 12 at precisely the maximum allowedtemperature. The flue gas temperature then drops to the exit temperatureas thermal energy is transferred to the working fluid. The flue gas hasapproximately constant thermal capacity as inferred by the nearly lineartemperature profile versus thermal energy transferred. A lower inlettemperature for the Hot-Gas into the HRSG required higher recyclingflowrate, which results in larger flue gas flowrates in the HRSG, aflatter temperature profile on the hot stream, and smaller temperaturedifferences between the streams of the HRSG. If thermal energy transferand pressure losses are independent of the flow conditions in the HRSG,then lower inlet temperature, leading to smaller temperature differencesand larger recycling flow rates, are clearly unfavorable regarding bothexchanger area and the CPR. However, the heat transfer, pressure losses,and flow conditions of the flue gas are not independent, so larger flowsand smaller entry temperatures might be favorable in some cases,especially when CPR is of a higher priority than area, since largerflowrates may contribute in smaller HRSG pressure losses.

The third profile in FIG. 2 represents a theoretical operation whilerespecting the maximum temperatures constraint; this graph is only forillustrative purposes. The profile is achieved by an infinite number ofsplits, and an infinitesimal recycling to the inlet required to decreasethe temperature of the infinitesimal inlet combustion gas from 1550° C.to 800° C. The split combustion gas is introduced infinitesimally intothe HRSG 12 maintaining for as long as possible a constant temperatureequal to the maximum allowed. When all the combustion gas is introduced,the temperature profile is only infinitesimally flatter than that ofProfile 1, and the two profiles seem indistinguishable.

The fourth profile in FIG. 2 represents an operation with a singlesplit. First, a certain amount of combustion gas is split, and justenough recycling to the inlet of the HRSG is used to obtain a Hot-Gastemperature of, for example, 800° C. The split is then introduced to theHRSG without any recycling at a point when the resulting flue gasmixture in the HRSG attains a temperature of, for example, 800° C.Compared to the conventional operation to Profile2, the split canprovide larger average temperature differences between the streams ofthe HRSG, therefore, smaller areas. Moreover, lower recycling flowrates,and possibly smaller CPR, are required since respecting the maximumtemperature constraints are attained not only by recycling but also bythe gradual heat transfer (note that pressure losses in the HRSG, whichmight increase, are another factor in determining the CPR). Lower fluegas flowrates can also be inferred from the steeper sloped of Profile4compared to those of Profile2. Introducing the split further downstreamand/or adding recycling to the split, neither of which are shown herebut considered later for optimization, result in a lower mixturetemperature inside the HRSG. Also, adding recycling to the split canallow earlier mixing positions while satisfying the maximum temperatureconstraint.

It can be proven geometrically that for a given split flowrate, thelargest temperature differences between the hot and cold streams areattained when the inlet and the mixing temperatures are at the maximumallowed and when there is no recycling to the split. Also, by comparingthe slopes of the temperature profiles, it can be proven that for anysplit flowrate operating with the mixing temperatures at the maximumpossible value minimizes the flowrate of the recycling streams. It istempting to say that for a given split flowrate the least arearequirements and the least recycling requirements are obtained when theconstraints of the maximum allowable temperatures are active; as theoptimization shows, in fact the area is minimized when the maximumtemperature constraints are active, however, this is not always true forthe CPR.

It also can be proven that any split operation with a certain totalamount of recycling, regardless of the number of splits, the splitsflowrates, or the mixing positions, can at most reach the bordersoutlined by a profile with no split and the same total amount ofrecycling introduced all at once to the inlet; as an example Profile3and Profile1 with an infinitesimal recycling to the inlet, respectively.

The purpose of the split disclosed herein is to reduce the capital costby utilizing smaller surface area of the heat exchanger and reduce theCPR by reducing pressure drops and/or recycling flowrates. The twoobjectives are neither equivalent, nor mutually exclusive, and have tobe accounted for simultaneously; smaller exchangers in general lead tolarger pressure losses, and the flow properties affect the heat transfercoefficient, the HRSG pressure drop, and the recycling losses. The HRSGarea may be calculated according to the logarithmic mean to temperaturedifferences and heat exchanger elements.

The CPR consists of two components: the first component of the CPR isthe power needed by the recycling fans to re-pressurize the recyclingstreams after experiencing pressure losses from the HRSG and therecycling pipes. The second component is the power needed to maintainthe main gas flow and overcome the HRSG pressure losses. The extracompression needed to overcome the main flow losses can be introducedprior to combustion or after the HRSG. For example, in standard coalpowerplants, the same fans or compressors that drive the inlet streamsto the combustor force the flow through the main stream pressure losses.But in pressurized OCC combustors, since a compressor is already presentafter thermal recovery and needed for the carbon sequestration unit(CSU), the compensation power requirement is less costly to be accountedfor by compressing a cooler stream with a lower flowrate post thethermal recovery.

As aforementioned, a single split is considered herein. The independentdecision variables are chosen to facilitate optimization, since theyallow satisfying some constraints by properly setting their ranges.Also, these variables are considered the simplest to monitor and to settheir desired values during operation. The optimization variables are:i-the split flowrate, {dot over (m)}_(Split 1,), ii-the temperature atthe inlet of the HRSG T_(Mix,0), iii-the flowrate of the recyclingstream to the split, {dot over (m)}_(Rec, 1), and finally iv-thetemperature of the flue gas in the HRSG after introducing the split,T_(Mix,1). The variables are illustrated in FIG. 1 and the ranges aredefined in Table 2.

Based on this choice of independent variables, important variables arenow dependent. For example, the flowrate of the recycling stream to theinlet, {dot over (m)}_(Rec,0), is dependent once the split flowrate andthe inlet temperatures are specified; i.e., the stream entering the HRSGis fully specified. Further, the position of introducing mixture of thesplit and its recycling in the heat exchanger,

${{Mix}\text{-}{Pos}} = \frac{A\mspace{11mu} {before}\mspace{11mu} {mixing}}{A\mspace{14mu} {total}}$

is dependent once the split flowrate, the flowrate of the recycling tothe split, and the mixing temperature are specified.

TABLE 2 Optimization variables, their ranges and the basecase defaultvalues, for a single split. Because most of the basecase variablesvalues are far from the optimum (zero split, no recycling to split, andno mixing within the exchanger), several initial guesses are implementedin order to exclude suboptimal convergence. The boundaries of the rangesof the temperature variables are set to avoid constraint violations.T_(Mix, 0) lower bound is set to the maximum temperature of the coldstreams, i.e. the reheat stream exiting the HRSG at 610° C.; also, theupper bound on mixing temperatures T_(Mix, 0) & T_(Mix, 1), are set tothe maximum allowable temperature in the HRSG 800° C.; and the lowerbound of T_(Mix, 1) is set to the temperature of the feedwater enteringthe HRSG 321.7° C. Num- Base-case and/or ber Variable Range defaultvalue 1 {dot over (m)}_(Split1) [0-300] kg/s 0 set to 100 kg/s 2T_(Mix, 0) [T_(FW, out)-T_(HRSG, max)] ° C. T_(HRSG, max) = 800° C. 3{dot over (m)}_(Rec, 1) [0-600] kg/s NA/0 4 T_(Mix, 1)[T_(FW, in)-T_(HRSG, max)] ° C. NA set to T_(HRSG, max) = 800° C.

The operation of the heat exchanger is subject to physical limitations.Introducing the split provides better performance while still satisfyingthese constraints. The temperatures of the streams inside the heatexchanger are a major concern. Defined by the metallurgic properties,the maximum temperature allowed in the HRSG is limited to THRSG max=800°C. In essence the constraint has to be satisfied at every point withinthe heat exchanger, but because the temperature monotonically decreasesalong the HRSG (apart form the mixing point), the constraint needs onlyto be imposed at the inlet and at the mixing position. For the addressedvalues of the input stream, the temperature of the cold stream enteringthe HRSG is safely above the acid condensation temperature of the fluegas, [11], therefore, there is no need to include the constraint on theminimum allowed temperature for avoiding condensation of acids in theflue gas or on the feedwater tubes in the flue gas. In contrast, it isbeneficial to include constrains on the MITA to avoid temperaturecrossovers and ensure a realistic operation. The physical limit of MITAis zero, but a value of 0.5° C is used to speed the optimizationprocess. In other words, the intuition that small MITAs are clearlyundesirable since they result in huge area requirement, is communicatedto the optimizer.

A coal powerplant utilizes different types of coals during its lifetime.The advantages of the split concept are not limited to the type of coalused; for any coal utilized for power generation, the HRSG-split can bedesigned accordingly in order to reduce the area and/or the CPR. Sinceduring the operation of the powerplant the utilized coal is expected tochange, it is extremely important that the optimal design of theHRSG-split for one coal is flexible to changing the coal; i.e., thedesign is also optimal for the other coal.

Assessing the flexibility of the HRSG-split in general requirescharacterizing the variables as design or operation valuables, [13, 14].Design variables are fixed upon design while operation variables canchange with the different operations of the HRSG. The four independentvariables for the optimization of the standalone HRSG-split TMIX0, {dotover (m)}Split1, TMix1, & {dot over (m)}Rec1, are all operationvariables, However, dependent variables of the standalone model, inparticular the split mixing position, Mix-Pos, and the resulting HRSGarea

$\frac{Aa}{Ab},$

are design variables and have to be common between the differentoperations. Luckily, based on the following approach, there is no needto reformulate the problem to include design variables as decisionvariables, which pose a lot of difficulties in solving for the objectivefunction which would require complicated numerical methods, and resultin large domains of infeasible operations.

The ideal flexibility is demonstrated herein by a much simpler approach;the same optimization above is performed on a standalone HRSG-split butwith the new specifications of the input streams which are relevant to acoal different from the original. The input streams specifications usedabove are a result of the pressurized OCC process designed for idealflexibility to uncertainties when operating with a typical bituminouscoal with composition similar to Venezuelan and Indonesian coals(referred to as CoalA), while the new streams' specifications resultfrom operating with a lower quality South African coal almost identicalto Douglas Premium or Kleincopje coal (referred to as CoalB), aspresented in [13], The multi-objective optimization of the HRSG-splitoperating with the streams conditions of CoalB results in a Pareto frontcurve very similar to that seen for the HRSG-split multi-objectiveoptimization (presented above) operating with the conditions of CoalA.More specifically, equal areas of the two Pareto fronts have identicalmixing positions, therefore, a given Pareto optimal design of theHRSG-split for one coal is also a Pareto optimal design for the second,therefore, the HRSG is thermodynamically ideally flexible. Moreover,equal areas between the original and new Pareto curves are a result ofequal weight vectors for the multi-objective optimization, therefore,the HRSG-split is also economically ideally flexible; determining themost profitable design does not require to consider the coaldistribution because any optimal design for one operation is optimal forthe other, and has the same tendencies/preferences towards each of thetwo objectives. A similar behavior to that of the coal variation isencountered for the other uncertainties. As a conclusion, the HRSG-splitis ideally flexible to uncertainties, and at least capable ofmaintaining the flexibility of the process it is incorporated in.

The application of the HRSG-split is not limited to the OCC process. Thesplit concept can be applied to any heat exchanger process that requiresa recycling stream to control the temperature of the main stream; forexample, in conventional boilers, both the PGR rates and the heatexchange area, particularly radiative, can be reduced. The concept canbe readily applied to new powerplants; moreover, the retrofit ofexisting plants in conceivable.

Although not shown here, the benefits of the HRSG-split in subcriticalpower cycles can have larger magnitudes than those obtained here for asupercritical. Recall that the smaller the temperature differencesbetween the streams, the larger the area required for the same amount ofheat transfer, and therefore, the exchanger area is directly related tothe value of the MITA. In the case study above, the feedwater conditionsare those from an optimized supercritical Rankine cycle, where the pinchin the HRSG is located at the cold end; the feedwater temperatureentering the HRSG is relatively large, by utilizing the FWHsregeneration, allowing higher rates of feedwater through the HRSG whilerespecting the MITA specifications, and thus larger flowrates throughthe expansion line to increase the power output and efficiency. Sincethe pinch is at the cold end, introducing the HRSG-split cannot avoidthe pinch because the flue gas temperature at the exit of the HRSGvaries only slightly due to the variations in the CPR. Also, thefeedwater temperature profile in the HRSG is smooth due to the absenceof phase transition. On the other hand, with subcritical feedwater, thetransition from the subcooled liquid state to the two-phase state ismarked by a sharp kink, and usually the pinch point occurs at thatlocation rather than at the cold end. Since in an HRSG-split thetemperature of the flue gas after mixing is larger than that of thebasecase operation, except very close to the exit of the HRSG where thetemperature might be slightly lower depending on the CPR, then thetemperature difference at the location of the pinch is larger than thatof the basecase. Now since in subcritical operations the pinch isalleviated, the reduction in area and pressure losses are significantlylarger compared to the supercritical scenario. Note that the increase inthe temperature approach due to the split in the suberitical processallows, upon process optimization, for even larger feedwater flowratethrough the HRSG which increases the power output and the efficiency;i.e., compared to the supercritical scenario, in a suberitical processthe savings on area and CPR are larger, and there is a possibility ofincreased power output and further in the efficiency.

The split concept disclosed herein is applicable to heat exchangers thatrequire recycling of the hot stream for temperature control, e.g., coalboilers and HRSGs. The concept proposes splitting the hot stream, whichhas a temperature higher than that allowed in the heat exchanger, beforeits dilution and its introduction into the heat exchanger. At the inletof the flue gas, a lower amount of dilution is required to control thetemperature of the now smaller fluid flowrate. The split fraction isthen introduced into the heat exchanger at an intermediate pointdownstream, increasing the temperature of the hot stream and enhancingthe temperature gradient of the heat exchange process. The concept isable to reduce the cost by reducing the area requirements and/orincrease the efficiency by decreasing the power required to compensatefor the pressure losses of the flue gas. The concept is illustrated in astandalone model of an HRSG in the context of a pressurized oxy-coalcombustion process. Multi-objective optimization is performed byconstructing the Pareto front of minimal area and minimal powerrequirements. Both the heat exchange area and the compensation powerrequirements are shown to be reduced compared to the conventionaloperation; in the illustrated case, the area can be reduced down to 0.63the original size, and the compensation power requirements can bereduced down to 0.82 the original requirements. The design and operationis not limited to new heat exchangers and retrofitting in consideredeasily possible because no changes in the internal structure of the heatexchanger is required.

Moreover, facing uncertainty in input parameters and operatingconditions, the split concept is shown to be ideally flexible andpreserves the flexibility of the process it belongs to.

Herein, the heat exchanger process is enhanced by a modification to thedesign of heat exchanger while holding the input streams and the totaltransferred thermal energy constant. However, the input streams to theexchanger are variables of the process it belongs to. Therefore, theoverall performance of the process and the performance of the HRSG andthe capital cost savings can be enhanced by simultaneous optimization ofthe HRSG-Split and the powerplant design.

Further details of the invention are included in “A Split Concept forHRSG (Heat Recovery Steam Generation) with Simultaneous Area Reductionand Performance Improvement,” Zehian and Mitsos, Vol. 71, pp. 421-431,Energy (2014).

The contents of the references listed herein are incorporated byreference in their entirety.

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1. Combustion system comprising; a combustor generating a combustiongas; a splitter for receiving the combustion gas and dividing thecombustion gas into first and second gas streams; a temperaturecontroller for receiving the first gas stream, mixing it with recycledgas and introducing it into a heat recovery steam generator at a desiredtemperature; a mixer for receiving the second gas stream, mixing it withrecycled gas and introducing it into the heat recovery steam generator;wherein cool gas exits the heat recovery steam generator, the cool gasforming the recycled gas.
 2. The combustion system of claim 1 whereinthe combustor is an oxy-coal combustor.
 3. The combustion system ofclaim 1 further including recycling fans to pressurize the recycled gasbefore mixing with the first and second gas streams.
 4. The combustionsystem of claim 1 wherein the mass flow rate of the second gas stream,temperature at an inlet of the heat recovery steam generator, and flowrate of the recycled gas are controlled to optimize the system.